1. Field of the Invention
This invention relates to a wire bonding apparatus (wire bonder). More specifically, the present invention relates to a leadframe support plate, particularly a heater plate, for a wire bonding apparatus.
2. Description of the Prior Art
In manufacturing electronic components such as ICs, use is made of a leadframe which has a plurality of die pads at a constant pitch and a plurality of leads associated with each of the die pads. An electronic chip, generally called "die", is mounted on a corresponding die pad of the leadframe and electrically connected to the associated leads by wire bonding which is performed by using a wire bonder.
There are three methods for performing a wire bonding operation. A first method is the ultrasonic bonding in which a metal wire (made of gold or aluminum) is bonded to a chip electrode by application of ultrasonic energy. A second method is the thermo compression bonding wherein a metal wire is bonded to a chip electrode by application of thermal energy under a compressive force. A third method is the ultra thermo bonding which is the hybrid of the first and second methods. The present invention is applicable to either of these methods.
For conveniently explaining the problems to be solved by the present invention, reference is now made to FIGS. 8 through 15 of the accompanying drawings showing the prior art.
FIGS. 8 and 9 show a prior art heater plate as a typical example of leadframe support plate. The heater plate indicated by reference numeral 11 has a frame supporting face 12 and a die-pad receiving recess 13 which has a die-pad supporting face (bottom face) 13a lower than the frame supporting face. The heater plate is made of stainless steel for example and provided with an unillustrated heating means.
As shown in FIGS. 10 and 11, a leadframe L used for manufacturing electronic components has a plurality of die pads L1 (only one shown) arranged at a constant pitch, and a plurality of leads L2 surrounding each die pad. The leadframe may be made by punching a metal sheet, 0.1-0.25 mm in thickness, which is made of an iron-nickel alloy, a copper alloy, or other metal.
As shown in FIG. 11, the die pad L1 is located slightly lower than the plane of the leadframe L for carrying an IC chip D (die) as a typical example of electronic chip. The die-pad receiving recess 13 of the heater plate 11 is provided due to such an arrangement of the die pad L1.
In operation for die-bonding, a relevant portion of the leadframe L is first placed on the heater plate 11, and a presser plate P is pressed against the leadframe, as shown in FIGS. 12 and 13. In this state, the die pad L1 of the leadframe L is supported on the die-pad supporting face 13a of the heater plate, whereas the leads L2 are supported on the frame supporting face 12 of the heater plate under the pressure of the presser plate P.
Then, a capillary head C with a penetrating gold wire W having a ball end W1 is lowered and pressed axially against a selected electrode pad D1 of the IC chip D, as shown in FIGS. 14a and 14b. In the thermo compression bonding, the heater plate 11 imparts heat to the IC chip D. As a result, the axial compression force applied to the wire W together with the thermal energy provided by the heater plate 11 causes the wire ball end W1 to be bonded to the relevant electrode pad D1. In the ultra thermo bonding, the capillary head C is further subjected to ultrasonic vibration to increase the wire bonding strength.
Then, the capillary head C is raised and brought to a selected one of the leads L2 while allowing supply of the wire W, as shown in FIG. 14c. The wire can be bonded to the relevant lead L2 in the same manner as described above.
Finally, the capillary head C is raised without allowing supply of the wire W, thereby causing the wire to be pulled off. A new ball end W1 is formed by using a torch T for melting the wire endwise, as shown in FIG. 14d.
The same method steps are repeated for performing a wire bonding operation with respect to the other leads L2 of the leadframe.
The prior art heat plate 11 is acceptable if the leadframe is prepared by an etching method to have no marginal burr. In reality, however, a punching method is preferred for its lower cost and higher productivity, and it is inevitable in this case that the prepared leadframe will have marginal burrs B, as shown in FIG. 15.
When the prior art heater plate 11 is used for supporting the leadframe L having such marginal burrs B, the burrs cause the die pad L1 to be slightly lifted from the die-pad supporting face 13a, as shown in FIG. 16. Therefore, the support for the die pad L1 becomes unstable, and it is difficult in this condition to properly apply the axial compression force and/or the ultrasonic energy to the IC chip D, thus resulting in poor wire bonding (low bonding strength, distortion of the wire ball W1, and so on).
It should be appreciated that each lead L2 of the leadframe L also has marginal burrs. However, the burrs of the lead does not pose any problem for two reasons. First, the lead is subjected to a coining process (as required for plating) prior to performing a wire bonding operation, and this coining process serves to reduce the burrs to a certain degree. Secondly, the supporting stability for the lead L2 is ensured by the use of the presser plate P, so that the presence of the burrs may be ignored.